Method of compensating for ink jet faults

ABSTRACT

A method of printing using an array of ink jet nozzles that are configured to print dots of a first size on a print medium includes the step of diagnosing a faulty ink ejector in the array. Dots of a second size are printed on the print medium with at least one functional ink ejector located adjacent to the faulty ink ejector, the second size being different to the first size.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 09/608,780 filed on Jun. 30, 2000, all of which isherein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to digital printing and more particularly toprinting using devices which eject ink onto the printed substrate.However, the invention is not limited to ink ejection devices and isalso applicable to laser, light emitting diode printers and to digitalphotocopiers.

BACKGROUND OF THE INVENTION

In ink ejection devices a printhead has an array of nozzles throughwhich ink is selectively ejected onto the substrate as the substratemoves relative to the printhead. The printhead may print by scanningacross the substrate to print horizontal bands or, if it is a full pagewidth printhead, it may pass along the length of the page. A blockednozzle will result in multiple horizontal blank lines, in the case of ascanning type printhead, or a blank vertical line in the case of a pagewidth printhead. Such blank lines are undesirable since they detractfrom the printed result.

The present invention provides a method of modifying the printing of animage so as to reduce or effectively eliminate the visual effect of oneor more such blocked nozzles apparent to the eye of an observer innormal use. However, the invention is applicable to other forms ofprinting where a device, whether passive or active, is repeatedly usedto produce dots of ink or the like on a substrate. The invention haspotential application to laser and LED type printers and photocopierswhere a fault in the imaging drum or light source can result in repeatedfaults in the image produced. As used above and throughout thedescription and claims the term image is to be understood to have abroad meaning and includes anything printed, such as text and linedrawings.

DISCLOSURE OF THE INVENTION

In one broad form the invention provides a method of modifying an imageto be digitally printed by a printing device to compensate for failureto correctly print dots of ink at specific locations, the methodincluding the steps of:

a) identifying said specific location or locations, and

b) adjusting the dot size of at least one a dot at a location adjacentor near to the respective specific location from that required by theimage data.

In another broad form the invention provides a printer having a row ofactivatable devices which, when activated, cause rows of dots to bedeposited onto a substrate and means to move the substrate relative tothe row of devices in a direction generally perpendicular to the row ofdots, said printer including:

a) means to determine if one or more of said devices is not operatingcorrectly; and

b) control means for analysing images or image data and for identifyinga specific location or locations where a dot of ink should be printed byactivation of a incorrectly operating device and for adjusting the sizeof dot produced by one or both of the devices on either side of thefailed device.

The incorrectly operating device will result in a defect line or linesin the image printed. Usually the incorrectly operating device willproduce no ink or not enough ink and so a blank or faint line will beproduced. To compensate adjacent ink dots will be caused to be largerthan required by the raw image data. Conversely if the incorrectlyoperating device is producing oversized ink dots, the dot size ofadjacent dots will be reduced.

Where a part of an image requires the incorrectly operating device todeposit a continuous or substantially continuous column of dots, thedots in adjacent columns are preferably all adjusted in size. If thereare a small minority of locations in the column of the incorrectlyoperating device which do not require ink, dots in adjacent columns mayor may not be adjusted in size.

Dots in more than the two adjacent columns may be adjusted in size. Dotsin adjacent columns may be adjusted in size only if they are withinpredetermined vertical or horizontal distances or both of one or morespecific location. For example only dots in the columns either side ofthe failed column may be adjusted in size but dots in those columns twoor three rows above and/or below the respective location may be adjustedin size.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall be better understood from the following non-limitingdescription of preferred embodiments and the drawings, in which

FIG. 1 shows a schematic illustration of a set of nozzles of an ink jetprinting head.

FIG. 2 shows a schematic illustration of an array of ink dots formed bythe printhead of FIG. 1 without fault correction operational.

FIG. 3 shows a schematic illustration of the same array of ink dots asin FIG. 2 formed by the printhead of FIG. 1, but with fault correctionoperational.

FIG. 4 shows a second schematic illustration of an array of ink dotsformed by the printhead of FIG. 1 without fault correction operational.

FIG. 5 shows a schematic illustration of the same array of ink dots asin FIG. 4 formed by the printhead of FIG. 1 but with fault correctionoperational.

DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

Referring to FIG. 1, a printhead 10 has an array of ink jet nozzles 12arranged in a singe line. For the purpose of explanation only 14 nozzlesare shown but in practice there will be from tens to thousands ofnozzles arranged in a line. Paper is passed underneath the printhead ina direction generally perpendicular to the line of ink jet nozzles, asindicated by arrow 14. The printhead may be a stationary or a movableprinthead. As the paper passes under the printhead the ink jet nozzles Ato N are selectively operated to cause an array of ink dots to be placedon the paper. This array is a series of columns and rows, the spacing ofwhich is dependent on the spacing of the inkjet nozzles and the minimumpaper feed step respectively. Whilst it is preferred that the horizontaland vertical spacing of the dots is the same, this is not necessarilyachievable due to the different sources of the spacing. The printheadmay be a page width printhead or a smaller printhead which scans acrossthe page to lay down a series of transverse bands of printing.

For the purposes of explanation it is assumed that inkjets a-g and i-ninclusive are operating correctly but, for whatever reason, inkjet h isnot operating correctly or at all. It is also assumed that thediagnostic systems of the printer, which will be well understood bythose skilled in the art, have detected that nozzle h is not functioningcorrectly. In most cases, a malfunctioning device will be partially ortotally blocked resulting in insufficient or no ink being deposited onthe paper.

Referring to FIG. 2, which schematically shows a portion of printingperformed by the printhead 10 without fault correction, there is a blankcolumn, labelled “h” corresponding to inkjet h, whilst columns a-g andi-n have been correctly selectively printed. This leads to one or moreblank lines appearing in the printing depending on whether the printhead10 is a full page width printhead or a scanning type printhead. Theunshaded circles numbered 16, 18, 20 and 22 represent drops of ink whichshould have been printed in column h but were not. FIG. 3 shows the sameimage printed by the printhead 10 but with fault correction according toan embodiment of the invention operational.

Referring to FIG. 3 the ink drops in columns g and i are caused to belarger than normal, as will be explained below. This reduces the amountof white space between the dots and between the columns g and i. Theeffect is that the un-printed column h is not apparent to the eye of theuser. When printing on A4 or letter size paper for reading at normaldistances, such as at 20 to 30 cm, the effect occurs at about 1600 dpiand upwards.

In the FIG. 3 print, only dots intended to be printed anyway in columnsg and i have been increased in size but it is within the scope of theinvention that extra dots of ink, whether of normal size or of adjustedsize, may be printed in the columns either side of the failed column inlocations when the image data does not require a dot. As seen in FIG. 3there are dots in the image at only about 50% of possible locations andso, even with oversize dots, there is still significant white space.This white space may be reduced by printing dots in vacant areas toreduce and/or break up the visual effect of the un-printed column

The area of each adjusted size dot is preferably increased by about 50%but this may be more or less, as needed. The oversize dots in the twocolumns may just touch dots in the same column. However, the sizeincrease may be less, such that the dots in each of the two columns ofdots do not join, or may be greater, such that adjacent dots overlap.

Where ink dots are required in column h at frequent intervals oversizedrops will be deposited continuously by nozzles g and i. It will beappreciated that when ink dots are deposited less frequently the dropsize of ink in columns g and i will only increase adjacent or near toareas where drops should occur in column h. These oversize drops mayextend into rows where no ink is intended in column h. Where ink is notintended in column h for large distances, preferably no oversize dropswill be created in columns g and i.

Referring to FIGS. 4 and 5 there are shown a second set of schematicprints without and with fault correction respectively. As seen in FIG.4, dots of ink are required, but not printed, in column h at rows 1, 2,3, 5 and 7, as indicated by open circles 30, 32, 34, 36 and 38. In FIG.5 dots in columns g and I are increased in size in rows above and belowun-printed dots 30, 32, 34, 36 and 38. Because there are more dots inthese columns than compared to the FIGS. 2 and 3 prints, the oversizedots overlap more and reduce the white space to a greater extent. Again,if desired, normal or oversize dots may be printed in vacant locations,such as column g, rows 1 and 5 and column I rows 2, 3 and 7.

In the case of ink ejection type printers, increased dot size isachieved by increasing the amount of ink ejected. In the case of thermalink ejection devices this may be achieved by increasing the duration ofthe heating current pulse. In the case of piezo electric ink ejectiondevices this may be by increasing the driving voltage or current tocause greater distortion or by increasing the pulse duration. Similarlywith mechanical type ink ejection devices the pulse width and/or drivingvoltage or current may be increased.

The invention is also applicable to situations where individual devicesare producing too much ink, in which case the adjacent devices may beadjusted to reduce the dot size of ink dots produced.

It will also be appreciated that this technique may be used with laserand LED printers and photocopiers and other types of digital printerswhere the placement of an ink dot is dependent on individual activationof a device or component. For example, an LED in a LED printer may failor there may be a defect in the photoconductive imaging drum of a laserprinter. In both cases, adjusting the size of adjacent dots can hide orreduce the visual effect of the defect in the device or component.

In the case of a laser or light emitting device type printer dot sizemay be modified by modulating the intensity and or total amount of thelight falling on the corresponding portion of the photoelectric imagingdrum.

1. A method of printing using an array of ink jet nozzles, each ink jetnozzle being configured to print dots of a first size on a print medium,the method comprising the steps of: diagnosing a faulty ink ejector inthe array; and printing dots of a second size on the print medium withat least one functional ink ejector located adjacent to the faulty inkejector, the second size being different to the first size.
 2. A methodas claimed in claim 1, which is carried out with a line of ink jetnozzles.
 3. A method as claimed in claim 2, wherein the step of printingdots of a second size is carried out with a pair of functional inkejectors each located on either side of the faulty ink ejector.
 4. Amethod as claimed in claim 1, wherein the step of printing the dots ofthe second size is carried out so that the second size is greater thanthe first size.
 5. A method as claimed in claim 4, wherein the step ofprinting the dots of the second size is carried out so that the secondsize is about 50% greater than the first size.
 6. A method as claimed inclaim 1 wherein, prior to the step of printing, the method furtherincludes the step of determining a required printing duration for the atleast one functional ink ejector to achieve said second size.
 7. Amethod as claimed in claim 6, that comprises the step of configuring theat least one functional ink ejector to print for the determinedduration.